The present application relates generally to sound transmission systems, and more particularly to a stereophonic sound transmission system compatible with present-day U.S. transmission standards, and to apparatus for use therein.
The transmission of stereophonic sound together with a conventional television picture transmission greatly enhances the realism and entertainment value of the program being transmitted. Various systems and apparatus have been proposed for such transmissions including various compatible subcarrier-type systems wherein left-plus-right (L+R) information is conveyed on the regular frequency-modulated sound channel of a composite television broadcast signal, and left-minus-right (L-R) information is conveyed on a subcarrier.
One such system, which was described in "Simultaneous Transmission of Two Television Sound Channels", NHK Laboratories Notes, Ser. No. 132, Feb. 1970, by Yasutaka Numaguchi, Yashitaka Ikeda, and Osamu Akiyama, conveyed L-R information on a single-sideband amplitude-modulated subcarrier frequency-modulated on the standard NTSC aural carrier. To simplify the synchronous detection required for demodulating the subcarrier in this system, the subcarrier was generated at a frequency of 23.625 KHz, or one and one-half times the 15.75 KHz horizontal scanning frequency of U.S. monochrome television broadcasts, enabling the missing subcarrier to be generated in the receiver L-R demodulator by sampling the horizontal deflection signal. This system was found to be unsatisfactory, primarily because of insufficient subchannel bandwidth, poor channel separation and ambiguity in development of the left (L) and right (R) audio signals at the receiver.
Another system proposed for stereophonic television sound transmission utilized a frequency-modulated subcarrier centered at 31.5 KHz, or twice the horizontal scanning frequency. This subcarrier, when frequency-modulated on the NTSC-standard aural carrier, provided an L-R bandwidth of 12 KHz. However, when it was attempted to add stereophonic demodulation capability to the 4.5 MHz sound channel of standard intercarrier-type television receivers to recover the L-R component, video signal component contamination resulted to an extent that satisfactory L-R audio signals could not be obtained without extensive modification of the receivers. Applying such subcarrier signals to conventional split-sound receivers, wherein separate intermediate frequency (IF) channels are provided for video and sound components, is not practical since the 41.25 MHz sound IF output of conventional modern TV tuners is above the range at which presently employed sound channel IF filters can achieve the required effectiveness.
Another system, which was proposed in U.S. Pat. No. 3,099,707 to R. B. Dome, utilized an amplitude-modulated suppressed-carrier subcarrier component, centered at 23.625 KHz to avoid interference with harmonics of the horizontal scanning signal, frequency-modulated on the sound carrier. To facilitate regenerating the subcarrier for demodulation purposes at the receiver a 39.375 KHz pilot signal was transmitted which, when combined with the 15.75 KHz horizontal scanning signal present in the receiver, resulted in generation of the suppressed 23.625 KHz carrier. This system did not provide satisfactory performance in that the bandwidth of the L-R channel was limited to 8 KHz with symmetrical side-bands. Attempting to increase available bandwidth by the use of assymmetrical sidebands was not practical because this introduced a principal harmonic of the horizontal scanning signal into the upper sideband of the L-R component.
Two additional systems, which differed from those proposed in the afore-described systems in that they employed a subcarrier centered at 31.5 KHz, or twice the horizontal scanning frequency, where shown in U.S. Pat. Nos. 3,046,329 to T. W. Reesor and 3,219,759 to R. B. Dome. The first system was a single-sideband system which necessitated the provision of complex filtering and demodulation circuitry in the receiver if unacceptably narrow L-R channel bandwidth was to be avoided. The second system, like other intercarrier systems, was susceptible to video signal component contamination in the sound channel. Furthermore, both of these systems required connection to or at least non-destructive sampling of the horizontal deflection signal within the receiver, necessitating in the case of an add-on adapter a modification of the receiver and the provision of an additional cable to a converter, thereby increasing installation cost and reducing the versatility of the converter.
In contrast, the system of the present invention utilizes an amplitude-modulated double-sideband suppressed-carrier 38 KHz subcarrier L-R component frequency-modulated on the main aural carrier together with a 15 KHz bandwidth L+R component and a 19 KHz pilot carrier. This forms a composite signal which is similar to that employed in stereophonic FM broadcasts in the United States. The use of this system simplifies the demodulation process at the receiver, and provides a signal which is compatible with conventional non-stereophonic sound television receivers. Also, the proposed system lends itself to use with self-contained converters of a design and construction which may be readily utilized in conjunction with existing monochrome or color television receivers.
Accordingly, it is a general object of the present invention to provide a new and improved system and apparatus for transmitting stereophonic sound information in conjunction with a standard television transmission.
It is another object of the present invention to provide a system and apparatus for transmitting stereophonic television sound which provides improved performance and which is less subject to interference from an accompanying video transmission.
It is another object of the present invention to provide receiving apparatus for receiving a subcarrier-type compatible stereophonic transmission which apparatus can be conveniently installed on an existing television receiver with minimal modifications to the receiver.
It is another object of the present invention to provide a converter for receiving a compatible stereophonic television sound transmission of the subcarrier-type which can be used in conjunction with a conventional stereophonic FM receiver.
It is another object of the present invention to provide apparatus for receiving a subcarrier-type stereophonic television sound transmission which can be economically constructed using standard commercially available components.
It is another object of the present invention to provide a system and apparatus for transmitting bilingual sound in conjunction with a standard television transmission.